An important factor in the building’s structural design is snow, specifically its weight.
According to the building’s location, dimensions, occupancy, roof’s shape, etc., the snow load to consider varies considerably. A good knowledge of the Construction Code and climatic conditions prevailing at the construction site is required to calculate this snow load.

For important buildings, the snow load calculation on the roof is shown in Part 4 of the Quebec Construction Code. This calculation uses the formula:

• ground snow load could be equal or exceed 1-in-50-years;
• different reduction factors to take into account are the wind exposure effects on the roofs (by default 0.8), its shape, its type of cladding and slope of its roof;
• a factor to consider is the additional accumulations due to the differences of the levels concerning the roofs, its obstacles, etc.;
• the rain load could be equal or exceed 1-in-50-years.

The ground snow load and the rain load are provided in the Code for different locations and based on the gathered historic data.

For housing and small buildings, Part 9 of the Construction Code simplifies the calculation by bringing together all the factors into one. The calculation becomes 55% of the snow load added to the rain load (or 45% if the roof has a width of 4.3 m or less). The load thus obtained is lower than the one calculated with the help of Part 4, considering that snow is more efficiently swept away by the wind on small roofs than on larger ones.

The National Farm Building Code of Canada is applied on farm buildings. The calculation of the snow load on the roofs is similar to the one of Parts 4 and 9 of the Construction Code (according to the size and number of storeys). Nevertheless, it uses the ground snow load and the rain load that can be equal or exceed 1-in-30-years. An additional reduction factor (0.8) could also be applied when it has a low human occupancy in the building.

Finally, a factor of safety of 50% shall be added to all snow loads to design the different structural elements that will support this load.

It is also important to keep in mind that snow removal on a roof cannot be taken into account to reduce the snow load when designing a roof.

The thickness of the snow on the roof is not a reliable indicator of the current load on the aforesaid roof. The snow may have different density according to its condition (see the Table below). For example, freshly fallen snow will be 7 times lighter than the ice and up to 5 times lighter than humid snow.

Having heavier snow and the presence of ice on the roof reduces the wind effect on it. Roofs that have been usually swept naturally by the wind can experience a heavier snow load when it is not swept by the wind due to the presence of ice or denser snow.

 Various Types of Precipitations Mass (kg/m³) Dry and freshly fallen snow 128 Dry and compact snow 192-400 Humid snow 432-641 Ice 897 Water (at 4 °C) 1 000

Table 1: Weights of various types of precipitations – Handbook of Steel Construction

In the User’s Guide – National Building Code, Part 4 of Division B, the specific gravity of the snow on the roofs may vary from 1.0 to 4.5 kN/m³ and the rainfall during winter may increase this data. It is mentioned that a specific gravity of 3 kN/m³ may be used to estimate the snow load at the end of winter.

Exceptional Accumulations

The distribution of the snow on the roof may not be uniform in some cases. Here below are a few examples of non-uniformity:

• Snow on the roof of a building may be swept away by the wind and increase the loads of the building’s roof beneath;
• The presence of the projection on the roof (e.g. a chimney or a mechanical unity) may increase the snow load around these;
• On a roof with 2 slopes, the snow may be swept away by the wind and a slope may have more snow than the other one;
• During the thaw period, the snow and ice gliding may create non-uniform loads on the roof and slide from a higher roof to a lower one.

All these particularities shall be taken into consideration while calculating the snow load of the building’s roof. To avoid bad surprises for the winters to come, it is imperative to build a building in the respect of Codes and Standards in force. In case of a collapse, an expert may enforce one’s rights if these Standards have not been respected.

By Arthur de coux, P.Eng.